1. Field of the Invention
This invention relates broadly in the field of electronic musical tone generators and in particular is concerned with providing a sliding polyphonic portamento effect.
2. Description of the Prior Art
Several of the conventional acoustical type orchestral musical instruments have the capability of producing continuous portamento effects. For a portamento effect the musical pitches are not changed abruptly in a transition from one note to the next, but rather glide smoothly in a continuous frequency transition between the pitches of the two notes. These instruments include the unfretted string instruments and the slide trombone. The novel musical effect of the portamento is especially useful in contemporary music and a variety of schemes have been used to imitate portamento transitions for keyboard electronic musical instruments.
A keyboard portamento system is disclosed in U.S. Pat. No. 4,103,581 entitled "Constant Speed Portamento." In the disclosed system each keyboard switch controls the pitch of the generated tone through a table of frequency numbers. The portamento effect of having the pitch of one note slide smoothly into the pitch of the next note is achieved by subtracting the frequency number of the new note to be generated from the frequency number controlling the frequency of the note currently being generated. A fraction of the difference is stored in an increment register and added over and over to the frequency number of the current note at a controlled rate until the frequency number equals the frequency control number of the new note. Thus the frequency transition from the first to the second note takes place in a fixed number of incremental steps, the transition time being independent of the difference in pitch between the two successive notes.
In U.S. Pat. No. 3,929,053, entitled "Production Of Glide And Portamento In An Electronic Musical Instrument" another version of a portamento system is described. The frequency transition is accomplished by successively adding and accumulating a frequency increment to an initial frequency number corresponding to the first note. Eventually the accumulated sum of the previous frequency number and the added increments will essentially equal the frequency number of the newly selected note. Thereafter tone production continues at the true pitch of the new note. In this fashion the time required for the portamento transition will depend upon the frequency separation between the two notes.
Both of the portamento effects produced by the systems described in the referenced patents produce frequency transitions having an almost mechanical-like precision in that once a speed control has been set the transition time is automatically predetermined. Moreover the start frequency and the end frequency are restricted to be true musical pitches instead of having the capability of deliberately ending or starting on "detuned" notes. The "lip smear" effect obtained by brass instrument musicians cannot be realistically imitated by these systems.
Electronic musical instruments have been built which obtain continuous frequency transition by the use of a slide-wire. The finger pressure on the wire is used to produce a variable voltage which is used to control the frequency of a voltage controlled oscillator. The slide-wire controlled portamento system offers a wide latitude of control for the musician and permits him to produce some remarkable and novel musical effects. This system suffers from mechanical problems in the implementation of the slide-wire contacts as well as in the frequency stability problems encountered in having a given position on the wire correspond to a specified frequency. The usual slide-wire portamento system is inherently monophonic in operation.